Certain materials, particularly semiconductor wafers, must be treated, manufactured, handled and otherwise processed with a very high level of precision. Various processing conditions can result in the creation of substantial stresses within the materials, and accordingly, evaluation of the surface curvature, tilt and roughness is useful in analyzing the potential for stress-related failure of such components during the manufacturing process. While measurement of the curvature of such surfaces for the purpose of determining stress levels has been attempted in a number of different forms, there is substantial room for improvement in existing techniques.
Typical of the current technology are the measurement apparatus and associated techniques disclosed in U.S. Pat. No. 5,912,738, issued to Chason, et al., and disclosing an apparatus for measuring the curvature of a surface using a plurality of parallel light beams which reflect from a surface and are directed to a detector to measure the separation of the reflected beams of light.
Additional methodologies are disclosed in U.S. Pat. No. 4,929,846, issued to Mansour, and U.S. Pat. No. 4,291,990, issued to Takasu. Further, an understanding of mechanical stresses in epitaxial films is discussed by Schell-Sorokin and Tromp, in Physical Review Letters, Vol. 64, number 9, Feb. 26, 1990.
Existing technology is adequate, in some circumstances, on smaller samples, and in conditions where it is not necessary to measure a selected area of a larger surface. However, even with the existing technology, there are serious limitations in that the existing technology is incapable of measuring the degree of tilt of a target surface, and there are no provisions in the current art for monitoring the measurement process and providing appropriate feedback. Likewise, current technology relies on large single arrays of parallel beams which are expensive to produce and require a corresponding increases in size of associated equipment to generate the necessary arrays.
Our invention overcomes these and other limitations in existing art, by providing a device which works well on large surfaces using a small array, incorporates unique feedback control, allows the measuring of a plurality of selected areas utilizing a repositionable sample stage, and which is capable of evaluating tilt and roughness as well as curvature.